This invention relates to tension members for erecting structures, and more specifically, to tension members for use in bridge building.
Mankind has used bridges of many types for hundreds of years to span streams, rivers, valleys etc. There are three basic types of bridges in use today--beam bridges, suspension bridges and arch bridges. Beam bridges are also known as girder bridges and simply rest on a number of supports. Suspension bridges utilize cables which are in tension and which exert a pull on their end abutments or supports. Finally, arch bridges utilize an arched compression member which thrusts outwardly on the end abutments. In the arch bridge design, as with other designs, however, both compression ad tension members are present in the construction. Further, the three basic types of bridges may be varied and combined to form different designs and construction. One variation is the cable-stayed bridge, which is currently popular in Japan and Europe. The cable-stayed bridge uses girders extending between vertical concrete pylons. The pylons extend vertically upwardly from the traveling deck of the bridge and are used to support a number of spaced apart cable stays. One end of the cable stays is secured to the pylons and the opposite end of the cable stays is secured to the deck, thus providing additional support for the deck.
In cable-stayed bridges, the cables stay currently in use are steel strands which are coated with a thin protective coating. The stays are prestressed to a known amount. It has been reported, however, that the cable stays currently in use are susceptible to fatigue failure due to vibrations caused by wind as well as traffic loads. In addition, sagging of cable stays under gravity loads may take place. Thus, an alternative tension member is needed that will overcome these drawbacks.
Further, in erecting structures, and particularly bridges, the choice of building materials is largely between steel and concrete. This distinction is not absolute, however, because nearly all concrete bridges include a large amount of steel as reinforcement, and the majority of steel bridges have concrete bridges.
In selecting between materials, the cost of materials as well as their load bearing characteristics are considered. Concrete is typically the cheapest serviceable material for the job and has good compressive strength characteristics. On the other hand, steel is substantially more expensive but has increased tensile strength as compared to concrete Because of these characteristics, concrete is typically used for members in compression and steel is typically used for members in tension. The disadvantage of concrete is its low tensile strength, which often necessitates the addition of reinforcement members, typically made of steel. Thus, the use of reinforced concrete in bridges dates to the later 1800s.
An alternative method of reinforcing concrete to increase its tensile strength involves stretching the reinforcement members before concrete is poured around them. When the stretching force is related from the reinforcement members, the resulting reinforced concrete member is prestressed by an equivalent compression. The reinforced concrete member will thereafter have an increased resistance to tension up to the point at which the added load exceeds the amount of prestressing force. Thus, it is known to increase the tensile strength of concrete by using prestressed reinforcing members. However, there remains a need for economically further increasing the tensile strength of the concrete members used in erecting structures such as bridges.
As stated above, concrete members used in erecting structures are known for their compressive strength characteristics. In building bridges, concrete members are used as compression members. While these concrete members have good compressive strength, there does exist a need for compression members that have greater compressive strength characteristic than provided by concrete members alone. To address this need, existing concrete columns in buildings have been retrofitted with a surrounding steel jacket. The jacket provides increased compressive strength characteristics to the overall member by providing a body which resists the lateral expansion exerted by compressive forces on the concrete. However, this technology has been limited to erecting buildings and has been used in earthquake prone regions of the world.
Therefore, a compression member for a bridge which takes advantage of the increased compressive strength and ductility of concrete by confining the lateral expansion of the concrete with a surrounding body made of steel or other material is needed for use in the bridge building industry. Still further, a tension member for use in bridge construction is needed that increases the tensile strength characteristics of a concrete member by incorporating high strength prestressing tendons, and steel tubing around the concrete which have superior tensile strength characteristics. Further yet, an alternative tension member is needed for use in a cable-stayed bridge construction that is less susceptible to fatigue failure caused by wind and vibration and that is less susceptible to sagging.